Synchronous motors need a direct current source to power the field (rotor) winding. Gates B128 Synchronous motors with brushless excitation system have a rotating exciter, usually located in a housing at the rear of the motor. Synchronous motors with brushless excitation have a low maintenance cost due to the fact that they do not have brushes and because they do not have electrical contacts, which eliminates the possibility of sparking, the synchronous motors with excitation of the brushless type are recommended for applications in special areas with an explosive atmosphere.
The exciter acts as an alternating current generator where the rotor that is situated on the motor shaft has a three-phase winding and the stator is formed by alternating north and south poles fed by an external direct current source. The three-phase rotor winding is connected to a bridge of rectifier diodes. The voltage caused in the rotor is rectified and used to power the field winding of the motor. The amplitude of this field current can be controlled through the rectifier that will power the field of the exciter stator.